-Availability of basic information on weed biology is an essential tool for designing integrated management programs for agricultural systems. Thus, this study was carried out in order to calculate the base temperature (Tb) of southern sandbur (Cenchrus echinatus), as well as fit the initial growth and development of the species to accumulated thermal units (growing degree days -GDD). For that purpose, experimental populations were sown six times in summer/autumn conditions (decreasing photoperiod) and six times in winter/spring condition (increasing photoperiod). Southern sandbur phenological evaluations were carried out, on alternate days, and total dry matter was measured when plants reached the flowering stage. All the growth and development fits were performed based on thermal units by assessing five base temperatures, as well as the absence of it. Southern sandbur development was best fit with Tb = 12 o C, with equation y = 0,0993x, where y is the scale of phenological stage and x is the GDD. On average, flowering was reached at 518 GDD. Southern sandbur phenology may be predicted by using mathematical models based on accumulated thermal units, adopting Tb = 12 o C. However, other environmental variables may also interfere with species development, particularly photoperiod. Keywords:Cenchrus echinatus, weed, biology, growing degree days, modeling. RESUMO -A disponibilidade de informações básicas sobre a biologia de plantas daninhas é uma ferramenta essencial para a elaboração de programas de manejo integrado em sistemas agrícolas. Nesse sentido, este trabalho foi desenvolvido com o objetivo de calcular a temperatura basal (Tb) do capim-carrapicho (Cenchrus echinatus), bem como ajustar o crescimento e o desenvolvimento inicial da espécie às unidades térmicas acumuladas (graus-dia). Para isso, foram realizadas seis semeaduras de populações experimentais em condição de verão/outono (fotoperíodo decrescente) e seis semeaduras em inverno/primavera (fotoperíodo crescente

-This work was carried out with the objective of evaluating growth and development of sourgrass (Digitaris insularis) based on days or thermal units (growing degree days -GDD). Two independent trials were developed aiming to quantify the species' phenological development and total dry matter accumulation in increasing or decreasing photoperiod conditions. Plants were grown in 4 L plastic pots, filled with commercial substrate, adequately fertilized. In each trial, nine growth evaluations were carried out, with three replicates. Phenological development of sourgrass was correctly fit to time scale in days or GDD, through linear equation of first degree. Sourgrass has slow initial growth, followed by exponential dry matter accumulation, in increasing photoperiod condition. Maximum total dry matter was 75 and 6 g per plant for increasing and decreasing photoperiod conditions, respectively. Thus, phenological development of sourgrass may be predicted by mathematical models based on days or GDD; however, it should be noted that other environmental variables interfere on the species' growth (mass accumulation), especially photoperiod.Keyword: Digitaria insularis, growing degree days, phenology, dry matter, biology. RESUMO -Objetivou-se neste trabalho avaliar o crescimento e o desenvolvimento do capim-amargoso (Digitaria insularis) com base em dias ou unidades térmicas acumuladas (graus-dia -GD). Para isso, dois experimentos independentes foram realizados visando quantificar o desenvolvimento

-This work was carried out with the objective of evaluating the growth and development of honey weed (Leonurus sibiricus) based on days or thermal units (growing degree days). Thus, two independent trials were developed to quantify the phenological development and total dry mass accumulation in increasing or decreasing photoperiod conditions. Considering only one growing season, honey weed phenological development was perfectly fit to day scale or growing degree days, but with no equivalence between seasons, with the plants developing faster at increasing photoperiods, and flowering 100 days after seeding. Even day-time scale or thermal units were not able to estimate general honey weed phenology during the different seasons of the year. In any growing condition, honey weed plants were able to accumulate a total dry mass of over 50 g per plant. Dry mass accumulation was adequately fit to the growing degree days, with highlights to a base temperature of 10 o C. Therefore, a higher environmental influence on species phenology and a lower environmental influence on growth (dry mass) were observed, showing thereby that other variables, such as the photoperiod, may potentially complement the mathematical models.Keywords: Leonurus sibiricus, growing degree days, phenology, dry mass, biology RESUMO -Este trabalho foi desenvolvido com o objetivo de avaliar o crescimento e o desenvolvimento do rubim (Leonurus sibiricus) com base em dias ou unidades térmicas (graus dia). Para isso, dois experimentos independentes foram desenvolvidos

-This work was carried out with the objective of elaborating mathematical models to predict growth and development of purple nutsedge (Cyperus rotundus) based on days or accumulated thermal units (growing degree days). Thus, two independent trials were developed, the first with a decreasing photoperiod (March to July) and the second with an increasing photoperiod (August to November). In each trial, ten assessments of plant growth and development were performed, quantifying total dry matter and the species phenology. After that, phenology was fit to first degree equations, considering individual trials or their grouping. In the same way, the total dry matter was fit to logistic-type models. In all regressions four temporal scales possibilities were assessed for the x axis: accumulated days or growing degree days (GDD) with base temperatures ( ).Keywords: Cyperus rotundus, growing degree days, phenology, base temperature, modeling. y = 37,678/(1+(x/509,353) -7,047 ). RESUMO -Este trabalho foi desenvolvido com o objetivo de elaborar modelos matemáticos para previsão do crescimento e desenvolvimento da tiririca (Cyperus rotundus) com base em dias ou unidades térmicas acumuladas (graus-dia). Para isso, dois experimentos independentes foram desenvolvidos: o primeiro com fotoperíodo decrescente (março a julho) e o segundo com fotoperíodo crescente (agosto a novembro). Em cada experimento, foram realizadas dez avaliações de crescimento